[clang] Remove intrusive reference count from `DiagnosticOptions` (#139584)

The `DiagnosticOptions` class is currently intrusively
reference-counted, which makes reasoning about its lifetime very
difficult in some cases. For example, `CompilerInvocation` owns the
`DiagnosticOptions` instance (wrapped in `llvm::IntrusiveRefCntPtr`) and
only exposes an accessor returning `DiagnosticOptions &`. One would
think this gives `CompilerInvocation` exclusive ownership of the object,
but that's not the case:

```c++
void shareOwnership(CompilerInvocation &CI) {
  llvm::IntrusiveRefCntPtr<DiagnosticOptions> CoOwner = &CI.getDiagnosticOptions();
  // ...
}
```

This is a perfectly valid pattern that is being actually used in the
codebase.

I would like to ensure the ownership of `DiagnosticOptions` by
`CompilerInvocation` is guaranteed to be exclusive. This can be
leveraged for a copy-on-write optimization later on. This PR changes
usages of `DiagnosticOptions` across `clang`, `clang-tools-extra` and
`lldb` to not be intrusively reference-counted.

1 files changed, 9 insertions, 5 deletions

diff --git a/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp b/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
index d0b855f..99212b8 100644
--- a/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
+++ b/clang/lib/Frontend/CreateInvocationFromCommandLine.cpp
@@ -31,11 +31,15 @@ std::unique_ptr<CompilerInvocation>
 clang::createInvocation(ArrayRef<const char *> ArgList,
                         CreateInvocationOptions Opts) {
   assert(!ArgList.empty());
-  auto Diags = Opts.Diags
-                   ? std::move(Opts.Diags)
-                   : CompilerInstance::createDiagnostics(
-                         Opts.VFS ? *Opts.VFS : *llvm::vfs::getRealFileSystem(),
-                         new DiagnosticOptions);
+  std::optional<DiagnosticOptions> LocalDiagOpts;
+  IntrusiveRefCntPtr<DiagnosticsEngine> Diags;
+  if (Opts.Diags) {
+    Diags = std::move(Opts.Diags);
+  } else {
+    LocalDiagOpts.emplace();
+    Diags = CompilerInstance::createDiagnostics(
+        Opts.VFS ? *Opts.VFS : *llvm::vfs::getRealFileSystem(), *LocalDiagOpts);
+  }
 
   SmallVector<const char *, 16> Args(ArgList);